Today the drive for miniaturization is exemplified by the cellular telephone industry. A few years ago large phones that allowed you to stay in touch were widely accepted. Today a cellular telephone must fit easily in the palm of your hand, even for a small woman or a child. The phone is also not for just keeping in touch. It has wireless internet access, a camera with video recording, embedded games and the ability to store the number for every person that you have ever dialed accompanied by their picture. In order to achieve this level of function packed into a very small space, package technology has made dramatic changes to enable the innovation.
Chip-on-board and board-on-chip (BOC) techniques are used to attach semiconductor dies to an interposer or other carrier substrate such as a printed circuit board (PCB). Attachment can be achieved through flip chip attachment, wire bonding, or tape automated bonding (“TAB”). Flip chip attachment typically utilizes ball grid array (BGA) technology. The BGA component (die) includes conductive external contacts, typically in the form of solder balls or bumps, arranged in a grid pattern on the active surface of the die, which permit the die to be flip chip mounted to an interposer or other carrier substrate (e.g., PCB).
In a flip chip attachment, the balls of the BGA component are aligned with terminals on the carrier substrate, and connected by reflowing the solder balls. The solder balls can be replaced with a conductive polymer that is cured. A dielectric under-fill is then interjected between the flip chip die and the surface of the carrier substance to embed the solder balls and mechanically couple the BGA component to the carrier substrate.
Wire bonding and TAB attachment generally involve attaching a die by its backside to the surface of a carrier substrate with an appropriate adhesive (e.g., epoxy) or tape. With wire bonding, bond wires are attached to each bond pad on the die and bonded to a corresponding terminal pad on the carrier substrate (e.g., interposer). With TAB, ends of metal leads carried on a flexible insulating tape such as a polyimide, are attached to the bond pads on the die and to the terminal pads on the carrier substrate. A dielectric (e.g., silicon or epoxy) is generally used to cover the bond wires or metal tape leads to prevent damage.
High performance, low cost, increased miniaturization of components, and greater packaging density of integrated circuits have long been goals of the computer industry. One method of increasing integrated circuit density while reducing package size and height is to stack dies vertically. Different approaches to packaging have been pursued to provide stacked die devices.
One such example of a stacked die to lower wire bond loop height is to mount a flip chip die on a chip-on-board (“FC-on-chip”). The package includes a flip chip die mounted via solder bumps with the active surface facing down onto the active surface of a bottom die (chip-on-board), which in turn, is mounted with an adhesive tape or paste onto an interposer substrate. Bonding wires connect the bond pads on the bottom die to lead or trace ends on the interposer. The interposer could include solder balls for mounting the encapsulated package (component) onto a substrate, e.g., motherboard or PCB.
Flip chip attachment has provided improved electrical performance and allowed greater packaging density. However, developments in ball grid array technology have produced arrays in which the balls are made smaller and with tighter pitch. As the balls become smaller and are set closer together, it poses problems for the mutual alignment of the conductive bumps on the flip chip die with the bond pads on the bottom die, requiring a metal reroute or redistribution layer (RDL) disposed as an intermediate layer on the surface of the bottom die. The RDL effects an electrical interconnection (redistribution) between the bond pads on the flip chip die to the bond pads on the bottom die for die attachment and wire bonding to the substrate.
Fabricating an FC-on-chip can also lead to high costs and process difficulties. For Example, a flip chip mounting device is required to accurately align the top die to the bottom die. Another drawback is that damage can occur to the active surface of the bottom die during an under-filling process onto the active surface, and a molding filler can fail to flow into voids between the dies if the gap is too small.
Thus, a need still remains for a wafer level chip scale package. In view of the enormous demand for smaller and more intelligent electronic devices, it is increasingly critical that answers be found to these problems. In view of the ever-increasing commercial competitive pressures, along with growing consumer expectations and the diminishing opportunities for meaningful product differentiation in the marketplace, it is critical that answers be found for these problems. Additionally, the need to save costs, improve efficiencies and performance, and meet competitive pressures, adds an even greater urgency to the critical necessity for finding answers to these problems.
Solutions to these problems have long been sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.